feat(#23): make several fixes to meet quality standarts

src/main/java/org/eolang/opeo/CompileMojo.java

@@ -36,7 +36,7 @@
  * @since 0.1
  */
 @Mojo(name = "compile", defaultPhase = LifecyclePhase.PROCESS_CLASSES)
-public class CompileMojo extends AbstractMojo {
+public final class CompileMojo extends AbstractMojo {
     @Override
     public void execute() {
         Logger.info(this, "opeo-maven-plugin: started compiling EO into low-level representation");

src/test/java/org/eolang/opeo/CompileMojoTest.java

@@ -0,0 +1,43 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016-2023 Objectionary.com
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package org.eolang.opeo;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit test for {@link CompileMojo}.
+ *
+ * @since 0.1
+ */
+class CompileMojoTest {
+
+    @Test
+    void createsMojoWithoutProblems() {
+        Assertions.assertDoesNotThrow(
+            () -> new CompileMojo().execute(),
+            String.format("Can't create %s mojo instance and execute it", CompileMojo.class)
+        );
+    }
+}